1. Field of Invention
The present invention relates to a microphone, and more particularly, to a capacitive microphone and method for making the same.
2. Related Art
Most micro capacitive microphones are manufactured through film processes. The remaining stress on the film limits the sensitivity of the microphone to a great extent, which can be released effectively by way of single-end support. However, the microphone is a structure with the surrounding parts being fastened during operation, so additional fastening structure designs are required.
As for U.S. Pat. No. 6,535,460, the backplate is located at the top of the structure, and contacts with the polysilicon diaphragm via the supporting structure. The backplate must be a nonconductor, and another layer of conductive material is required to form the top electrode. The materials of the backplate and the sacrificial layer must have high etching selectivity. The backplate must control its remaining stress, to avoid being initially deformed. The processing of this manufacturing method is relatively complex.
As for U.S. Pat. No. 5,146,435, a thick plate structure suspended by a spring structure is used instead of the conventional diaphragm, to form a parallel plate movement under the effects of sound pressure. However, the hollow portion of the twist beam spring will result in poor low frequency functionality of the microphone, and the twist beam may be deformed due to the stress gradient of the material.